The air brake system for a vehicle, for example, a bus, a truck, a trailer, other heavy machinery vehicles, and the like is formed of a brake shoe, and a drum assembly which can be driven by an actuator assembly operated by selectively supplying compressed air.
The air brake actuator is formed of a service brake actuator configured to operate a brake in an ordinary driving state by supplying compressed air, and a spring type emergency brake actuator which operates the brake if the air pressure becomes low.
The emergency brake actuator is equipped with, for example, a strong compressive spring which is able to forcibly apply a braking force if the air is eliminated.
These types of springs are collectively called a spring brake.
The pneumatic brake actuator may be a piston type or a diaphragm.
In the diaphragm type, the brake actuator and two pneumatic diaphragm brake actuators, in general, are disposed in a tandem vehicle configuration. There are provided a pneumatic service brake actuator configured to drive an ordinary operative brake, and a spring brake actuator which is able to allow for the parking of a vehicle or an emergency brake.
The service brake actuator and the spring brake actuator includes a housing having an elastic diaphragm which divides the inside of the housing into two fluid chambers.
Meanwhile, the piston brake actuator operates based on the same principle except for the diaphragm, and a piston reciprocates inside a cylinder which is configured to drive the parking brake of a vehicle.
In the conventional service brake actuator, a service brake housing is divided into a service chamber and a pushrod chamber.
The service chamber is connected to the source of compressed air in such a way that a fluid movement is available, and the push rod chamber is equipped with a pushrod and is connected to the brake assembly, and the push rod reciprocates between the inside and outside of the housing, thus locking or unlocking the operative brake which is configured to input the compressed air into the compressed chamber or output it from the compressed chamber.
In the ordinary spring brake actuator, the spring brake housing is divided into a pressure chamber and a spring chamber.
The opposite ends of the piston position at a spring chamber between the strong compressive spring and the diaphragm.
The known actuator rod extends to the pressure chamber through the pressure plate and the diaphragm and extends through a separation wall which is able to separate the spring actuator from the service brake actuator.
The end of the actuator is connected to the service chamber of the service brake actuator, so a fluid can flow between them.
If the parking brake is operated, the pressure of the spring brake actuator is discharged from the pressure chamber, and the strong compressive spring pushes the piston toward the separation wall between the spring brake actuator and the service brake actuator.
At this position, the actuator rod connected to the pressure plate operates the parking or emergency brake and is pushed so that the vehicle cannot be forcibly moved.
In order to unlock the parking brake, the pressure chamber is closed from the atmosphere, and the compressed air allows to expand the pressure chamber and to move the diaphragm and to move the pressure plate toward the opposite ends of the spring brake actuator housing, by which the pressure plate is inputted into the pressure chamber of the spring brake actuator which is able to compress the strong compressive spring.
If the spring to which a large compressive force is applied, is compressed in this spring brake actuator, the volume of the pressure chamber increases, and the volume of the spring chamber decreases, for which the pressure in the spring chamber increases, thus causing a problem.
Since the compressive air system for a heavy equipment vehicle is configured to operate at the maximum standard pressure, the pressure generated in the spring chamber may be multiplied.
The increases in the pressure of the engaged spring and the air pressure in the spring chamber cannot approach the maximum value at which the brake can appropriately operate.
If the combined force related with the pressure of the spring and the pressure generated in the spring chamber approaches the force which has been applied to the maximum, the brake may become a state where it cannot be unlocked, whereby the brake may be partly unlocked or may be very slowly unlocked.